This invention is directed toward blow molding containers from tubular parisons and more particularly to blow molding containers where the thermoplastic is multiaxially molecularly oriented during forming.
It is known to molecularly orient thermoplastics in systems wherein such materials are being blow molded into hollow articles such as containers. Such molecular orientation is highly desirable, when the thermoplastic is of such a nature that orientation can be developed therein, since it can represent an attractive route toward improving the strength properties of the finished container. Alternative ways of improving strength involve changing the chemical structure of the thermoplastic during synthesis by means of strength imparting modifiers, or by increasing the wall thickness of the finished article. The former alternative may undesirably affect other properties of the material while the latter is rather costly, especially when the thermoplastic is one of the more recently developed low permeability nitrile based resins known as LOPAC, a registered trademark of Monsanto Company. Accordingly it would seem desirable to develop such orientation during the forming operation since in any event the plastic must be molded in some way into the shape of the finished article.
Generally speaking, to molecularly orient an orientable thermoplastic, its temperature must be within a particular range at the time the material is stretched to develop the orientation, this range being lower than the temperature at which the thermoplastic is extruded yet considerably above room temperature.
Prior art teachings for developing such molecular orientation, such as that disclosed in U.S. Pat. No. 3,470,282, have been based on injection molding the thermoplastic followed by temperature conditioning and stretching axially and radially in the blow mold. Though this technique is generally satisfactory, it is not without its shortcomings. As is also known, it is highly desirable to develop a preselected distribution of wall thickness in the thermoplastic prior to final blowing in order to compensate for the different amount of stretching that occurs during blowing and consequently to achieve fairly uniform wall thickness in the finished article, or alternatively to provide a particular area of the finished article which has a different wall thickness than other areas for performance purposes. When the parison is formed by injection molding, the position of the entire injection mold surface corresponding to the parison shape must be controlled to obtain such a programmed parison whereas only the position of the much less extensive surfaces defining an annular outlet orifice need be controlled to vary the parison wall thickness when forming by extrusion. In addition, very high temperatures and pressures and consequently heavy equipment are required for injection molding, such temperature and pressure conditions possibly being deleterious in the sense of promoting degradation when the thermoplastic is of the heat sensitive variety. Also,when the thermoplastic is relatively viscous and stiff in flow at molding conditions, it may not be possible to conveniently inject such material into narrow injection molding cavities, or, put another way, the size of the container which is possible to mold from such materials may be restricted since the material will not flow sufficiently far into the relatively narrow, deep, injection mold without undesirably deflecting the inner core pin. Also, it has been found that in transferring an injection molded parison to the blow molding station, it has a tendency to undesirably shrink and curl, possibly due to the flow orientation developed in the thermoplastic during the injection molding step.
Other prior art techniques, such as that disclosed in U.S. Pat. No. 3,294,885 and other related patents, teach reheating the thermoplastic back up to orientation temperature after initial cooling and prior to stretching. Though such techniques are again considered generally satisfactory under certain conditions, if the parison is programmed such that the wall thickness is varied as discussed previously, it is difficult to achieve a relatively uniform temperature through the full wall of the varying thickness material without using a special zone type of heating approach, and if substantial temperature gradients exist across the parison thickness the extent of stretch and consequently of molecular orientation will vary, i.e. the hot thin sections will stretch greater than the cooler thicker sections and the desired orientation level and material distribution pattern in the finished container may not be attainable.
Other problems with prior art blow molding orientation tehcniques have occurred in the area of manipulating the thermoplastic while at orientation temperature, e.g. shaping the finish of the container and/or forming a welded seal. Special heated mold members and/or multi-arm sealing mechanisms such as that set forth in U.S. Pat. No. 3,430,290 have been used in the past.